1. Field of the Invention
This invention pertains in general to a signal transfer system for time division switching centers and in particular to a "line" signalling transfer system between at least one multiplex highway having access to a time division switching center including at least one multiregister provided in this center.
2. Description of the Prior Art
Setting up, holding and releasing of telephone calls, in particular, requires signal transmission between switching centers. In this respect, a distinction is usually made between "line" signalling which includes exchanges of signals generally concerning the state of the call or of the channels it uses between the centers and "register" signalling which includes exchanges of signals concerning the call destination (called subscriber number, class of call, etc. . . . ) and its routing situation (congestion, busy condition, free subscriber, etc. . . . ). This distinction is far from being general and depends on the telephone networks. It appears typically in networks using multifrequency signalling. Register signalling is exchanged as voice frequency signals during the call setting up period, line signalling concerns all the other signals exchanged during the whole time of the call.
Line signalling is usually exchanged between units permanently connected to the call channel, while register signalling requires complex units which are connected to the channel only on a temporary basis.
When a call uses a pulse code modulation (PCM) multiplex highway, line signalling is transmitted by a signalling channel, while the register signalling uses the speech channel.
The PCM highway of the 30 channel CEPT type is based on a 125 us recursive frame divided into 32 equal time slots. Each time slot permits transmission of 8 binary digits or bits. The first time slot of each frame is used for frame synchronization. The 17th time slot is used for the signalling channel. The other 30 time slots correspond to 30 speech channels. In the case of channel to channel signalling, the 17th time slots of 16 consecutive frames forming a multiframe permit transmission of the 30 speech channel line signalling. Out of the 16 8-bit combinations supplied by these 17th time slots of the multiframe, the first is used for multiframe synchronization, the other 15 each carry line signalling of two distinct speech channels and thus permit transmission of 4 bits per speech channel.
In a time division switching center in which this type of highway arrives, line signalling can be collected and injected at the center inputs by a line signaller controller serving as interface equipment between the PCM highways and the switching center central control unit. Register signalling, however, calls for a multiregister. At the call setting up stage, a speech channel is connected through the center to an access of a multiregister which receives and sends the voice frequency signals (in PCM form) of the register signalling and will therefore in this respect serve as interface equipment between the speech channel and the central unit.
U.S. Pat. No. 3,970,798 issued July 20, 1976 to G. Epenoy et al discloses a switching-processing node for use in a high speed data transmission system. The system separates the data from control signals upon receipt of a transmission. The data is then rejoined with the appropriate control signals. Other patents representative of the prior art are: U.S. Pat. Nos. 4,002,846; 3,997,728; 3,988,544; 3,984,642; 3,970,799; 3,952,162; 3,925,620 and 3,922,497.